Co2 reduction toward methane

ABSTRACT

An electrode of a chemical cell includes a substrate having a surface, an array of conductive projections supported by the substrate and extending outward from the surface of the substrate, each conductive projection of the array of conductive projections having a semiconductor composition for reduction of carbon dioxide (CO2) in the chemical cell, and a catalyst arrangement disposed along each conductive projection of the array of conductive projections, the catalyst arrangement including a copper-based catalyst and an iron-based catalyst for the reduction of carbon dioxide (CO2) in the chemical cell.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional applicationentitled “CO₂ Reduction toward Methane,” filed Dec. 9, 2019, andassigned Ser. No. 62/945,661, the entire disclosure of which is herebyexpressly incorporated by reference.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The disclosure relates generally to photoelectrochemical and otherchemical reduction of carbon dioxide (CO₂) into methane.

Brief Description of Related Technology

Solar-powered CO₂ reduction with water (H₂O) has been proposed as amechanism for reducing greenhouse gas (CO₂) emissions, whilesimultaneously converting renewable solar energy into storable,value-added fuels and other chemicals. The photoelectrochemical (PEC)route to CO₂ reduction combines light harvesting photovoltaic andelectrochemical components into a monolithically integrated device.

Among various products formed from PEC CO₂ reduction, the most reduced,methane, is highly energy-dense (ΔH_(c)°=891 kJ/mol). The storage,transportation, and combustion of methane are compatible with theexisting industrial infrastructure. Methane is thus an ideal solar fuel.Unfortunately, the production of methane involves complicatedeight-electron/proton coupling transfer, which is both kinetically andthermodynamically unfavorable.

A number of electrocatalysts including molecular complexes, enzymes,metals, and transition metal chalcogenides, have been developed for CO₂reduction. Among these materials, copper (Cu) is known to be astate-of-the-art electrocatalyst for producing methane from CO₂reduction. However, the use of Cu as a catalyst for PEC methanesynthesis has suffered severely from low current density, inferiorFaradaic efficiency, low turnover frequency, and high overpotential.This is because Cu, with a monofunctional site, generally possesses avery weak interaction with CO₂, and is not capable of concurrentlyactivating CO₂ molecules and stabilizing the subsequent reactionintermediates.

Binary catalysts of Cu with secondary metals and their derivatives haveemerged as a possible approach to enhance the performance of PEC CO₂reduction. For example, an oxide-derived Cu—Zn electrocatalyst exhibiteda remarkable enhancement on tunable syngas formation with a benchmarkturnover number of 1330 compared to Cu alone. Directed assembly of CuAunanoparticles on silicon nanowire photoelectrodes exhibited an evidentlyaccelerated CO₂-to-CO conversion with high selectivity of 80% at −0.2 V.A Cu—Zn alloy for selectively reducing CO₂ towards HCOOH exhibited aFaradaic efficiency of 79.11% through photoelectrocatalysis, which issuperior to either Zn or Cu. Nevertheless, these reported binary systemsare still not efficient enough to improve the interaction with CO₂ formethane synthesis from PEC CO₂ reduction.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, an electrode of achemical cell includes a substrate having a surface, an array ofconductive projections supported by the substrate and extending outwardfrom the surface of the substrate, each conductive projection of thearray of conductive projections having a semiconductor composition forreduction of carbon dioxide (CO₂) in the chemical cell, and a catalystarrangement disposed along each conductive projection of the array ofconductive projections, the catalyst arrangement including acopper-based catalyst and an iron-based catalyst for the reduction ofcarbon dioxide (CO₂) in the chemical cell.

In accordance with another aspect of the disclosure, a photocathode fora photoelectrochemical cell includes a substrate including a lightabsorbing material, the light absorbing material being configured togenerate charge carriers upon solar illumination, an array of conductiveprojections supported by the substrate, each conductive projection ofthe array of conductive projections being configured to extract thecharge carriers from the substrate, a plurality of catalyst particlesdisposed across each conductive projection of the array of conductiveprojections, each catalyst particle of the plurality of catalystparticles including copper, and a distribution of an iron-based catalystdisposed adjacent to the plurality of catalyst particles in aco-catalyst arrangement with the plurality of catalyst particles for thereduction of carbon dioxide (CO₂) in the chemical cell.

In accordance with yet another aspect of the disclosure, a method offabricating an electrode of an electrochemical system includes growingan array of conductive projections on a semiconductor substrate, eachconductive projection of the array of conductive projections having asemiconductor composition for reduction of carbon dioxide (CO₂) in theelectrochemical system, and depositing a catalyst arrangement along eachconductive projection of the array of conductive projections, thecatalyst arrangement including a copper-based catalyst and an iron-basedcatalyst for the reduction of carbon dioxide (CO₂) in the chemical cell.

In connection with any one of the aforementioned aspects, theelectrodes, systems, and/or methods described herein may alternativelyor additionally include or involve any combination of one or more of thefollowing aspects or features. The copper-based catalyst includes aplurality of copper nanoparticles. The iron-based catalyst includes adistribution of iron oxide disposed in a co-catalyst arrangement withthe copper-based catalyst. The copper-based catalyst is disposed betweenthe iron-based catalyst and the conductive projection. The copper-basedcatalyst and the iron-based catalyst are linked by a metallic bond. Thesubstrate includes a semiconductor material. The semiconductor materialis configured to generate charge carriers upon absorption of solarradiation such that the chemical cell is configured as aphotoelectrochemical system. The array of conductive projections areconfigured to extract the charge carriers generated in the substrate.Each conductive projection of the array of conductive projectionsincludes a respective nanowire. Each conductive projection of the arrayof conductive projections includes a Group III-V semiconductor material.The structure is planar. The semiconductor composition of the array ofconductive projections establishes a Schottky junction with the catalystarrangement. The catalyst arrangement has an iron-to-copper ratio ofabout 6.3 to 1. The copper-based catalyst may be partially oxidized. Thechemical cell is a thermochemical cell. An electrochemical systemincluding a working electrode configured in accordance with an electrodeas described herein, and further including a counter electrode, anelectrolyte in which the working and counter electrodes are immersed,and a voltage source that applies a bias voltage between the working andcounter electrodes. The bias voltage establishes a preference for thereduction of carbon dioxide (CO₂) at the working electrode towardmethane. The iron-based catalyst includes iron oxide. Each conductiveprojection of the array of conductive projections includes a respectivenanowire. Each catalyst particle of the plurality of catalyst particlesis configured as a copper nanoparticle. Each conductive projection ofthe array of conductive projections includes a Group III-V semiconductormaterial. A photoelectrochemical system including a working photocathodeconfigured in accordance with a photocathode described herein, andfurther including a counter electrode, an electrolyte in which theworking photocathode and the counter electrode are immersed, and avoltage source that applies a bias voltage between the workingphotocathode and the counter electrode. The bias voltage establishes apreference for the reduction of carbon dioxide (CO₂) at the workingelectrode toward methane. Depositing the catalyst arrangement includesimplementing a number of electrodeposition cycles. The number ofelectrodeposition cycles is about 10 cycles. Implementing the number ofelectrodeposition cycles includes immersing the array of conductiveprojections in a solution, the solution including a copper precursor andan iron precursor.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawingfigures, in which like reference numerals identify like elements in thefigures.

FIG. 1 is a schematic view and block diagram of an electrochemicalsystem having a working electrode with a co-catalyst arrangementdisposed along a plurality of nanowires for reduction of carbon dioxide(CO₂) into methane in accordance with one example.

FIG. 2 is a method of fabricating an electrode with a co-catalystarrangement disposed along a plurality of nanowires for reduction ofcarbon dioxide (CO₂) into methane in accordance with one example.

FIG. 3 depicts an energy bandgap diagram of a GaN nanowire with aCuFe-based co-catalyst arrangement for PEC reduction of CO₂ towardmethane.

FIG. 4 depicts scanning electron microscope (SEM) images of an electrodewith a plurality of GaN nanowires before and after deposition of aCuFe-based co-catalyst arrangement in accordance with one example inwhich implementation of 10 hours of electrodeposition at −1.2 V inCO₂-purged 0.5 M KHCO₃ aqueous solution under standard one-sunirradiation.

FIG. 5 depicts a side, schematic view of CO₂ adsorption and activationinvolving Fe₃O₆H₆/Cu(111).

FIG. 6 depicts scanning electron microscope (SEM) images (at 1 μm scaleand insets at 500 nm) of bare GaN nanowires on a Si substrate in Part(a), and of a CuFe co-catalyst arrangement on GaN nanowires in Part (b),as well as a STEM-HAADF image of a GaN nanowire modified with binaryCuFe catalyst in Part (c), elemental distribution mappings of Ga in Part(d), N in Part (e), Fe in Part (f) and Cu in Part (g), and XPSmeasurements of Cu 2p in Part (h) and Fe 2p in Part (i) in a CuFeco-catalyst arrangement on GaN nanowires projecting from a Si substrate.

FIG. 7 depicts graphical plots of photoelectrocatalytic performancemeasurements, including J-V curves in Part (a), Faradaic efficiencies inPart (b), Partial current density in Part (c), and CH₄ productivity inPart (d) of GaN nanowires on a Si substrate, Cu on GaN nanowires, ironon GaN nanowires, and CuFe on GaN nanowires, together with a curve inPart (a) that corresponds to CuFe on GaN nanowires under dark, as wellas variations of Faradaic efficiencies in Part (e) and turnoverfrequency in Part (f) for methane synthesis versus applied bias for CuFeon GaN nanowires, under the following experimental conditions:CO₂-purged 0.5 M KHCO₃ aqueous solution (pH of about 8), and one-sunillumination (AM 1.5 G, 100 mWcm⁻²).

FIG. 8 depicts graphical plots of free energy diagrams for CO₂ reductionon Cu(111) and Fe₃O₆H₆/Cu(111) under zero potential in Part (a) andapplied electrode potentials in Part (b), respectively, in whichU_(L)(CO₂) shows the potential-determining energy barriers that areovercome for methane production on Cu(111) and Fe₃O₆H₆/Cu(111),respectively, and respective spheres represent hydrogen, carbon, oxygen,copper, and iron.

FIG. 9 depicts graphical plots of J-V curves in Part (a) and Faradaicefficiency and productivity in Part (b) of CuFe/Si and CuFe on GaNnanowires supported by a Si substrate, together with a schematic diagramof spatial decoupling of CO₂ reduction from light absorption and chargecarriers separation over the CuFe on GaN nanowires in Part (c), underthe following experimental conditions: CO₂-purged 0.5 M KHCO₃ aqueoussolution (pH of about 8), and one-sun illumination (AM 1.5 G, 100 mWcm⁻²).

The embodiments of the disclosed electrodes, systems, and methods mayassume various forms. Specific embodiments are illustrated in thedrawing and hereafter described with the understanding that thedisclosure is intended to be illustrative. The disclosure is notintended to limit the invention to the specific embodiments describedand illustrated herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

Electrodes of photoelectrochemical and other chemical cells having aco-catalyst arrangement for reduction of carbon dioxide (CO₂) intomethane are described. Methods of fabricating photocathodes and otherelectrodes for use in photoelectrochemical and other chemical systemsare also described.

Although described herein in connection with electrodes having GaN-basednanowire arrays for PEC CO₂ reduction, the disclosed electrodes are notlimited to PEC reduction or GaN-based nanowires. A wide variety of typesof chemical cells may benefit from use of the nanowire-nanoparticleinterface, including, for instance, electrochemical cells andthermochemical cells. Moreover, the nature, construction, configuration,characteristics, shape, and other aspects of the structures on or towhich the nanowires and/or nanoparticles are deposited may vary. Thedisclosed electrodes, systems, and methods may also be directed to CO₂reduction products other than or in addition to methane, such as CO,CH₃OH, CH₄, C₂H₄, C₂H₅OH, and C₂H₆.

FIG. 1 depicts a system 100 for reduction of CO₂ into methane. Thesystem 100 may also be configured for other reduction reactions. Thesystem 100 may be configured as an electrochemical system. In thisexample, the electrochemical system 100 is a photoelectrochemical (PEC)system in which solar or other radiation is used to facilitate the CO₂reduction. The manner in which the PEC system 100 is illuminated mayvary. In thermochemical examples, the source of radiation may bereplaced by a heat source.

The electrochemical system 100 includes one or more electrochemicalcells 102. A single electrochemical cell 102 is shown for ease inillustration and description. The electrochemical cell 102 and othercomponents of the electrochemical system 100 are depicted schematicallyin FIG. 1 also for ease in illustration. The cell 102 contains anelectrolyte solution 104 to which a source 106 of CO₂ is applied. Insome cases, the electrolyte solution is saturated with CO₂. Potassiumbicarbonate KHCO₃ may be used as an electrolyte. Additional oralternative electrolytes may be used. Further details regarding oneexample of the electrochemical system 100 are provided below.

The electrochemical cell 102 includes a working electrode 108, a counterelectrode 110, and a reference electrode 112, each of which is immersedin the electrolyte 104. The counter electrode 110 may be or include ametal wire, such as a platinum wire. The reference electrode 112 may beconfigured as a reversible hydrogen electrode (RHE). The configurationof the counter and reference electrodes 110, 112 may vary. For example,the counter electrode 110 may be configured as, or otherwise include, aphotoanode at which water oxidation (4H₂O⇔2O₂+8e⁻+8H⁺) occurs.

The reduction of CO₂ to methane occurs at the working electrode 112 asfollows:

CO₂ reduction: CO₂+8H⁺+8e⁻⇔CH₄

To that end, electrons flow from the counter electrode 110 through acircuit path external to the electrochemical cell 102 to reach theworking electrode 108. The working and counter electrodes 108, 110 maythus be considered a cathode and an anode, respectively. FIG. 3 depictsan energy bandgap diagram of the reduction of CO₂ to methane inconnection with a working electrode having a GaN nanowire and aCuFe-based co-catalyst arrangement, as described below.

In the example of FIG. 1, the working and counter electrodes areseparated from one another by a membrane 114, e.g., a proton-exchangemembrane. The construction, composition, configuration and othercharacteristics of the membrane 114 may vary.

In this example, the circuit path includes a voltage source 116 of theelectrochemical system 100. The voltage source 116 is configured toapply a bias voltage between the working and counter electrodes 108,110. The bias voltage may be used to establish a ratio of CO₂ reductionto hydrogen (H₂) evolution at the working electrode, as describedfurther below. The circuit path may include additional or alternativecomponents. For example, the circuit path may include a potentiometer insome cases.

In some cases, the working electrode 108 is configured as aphotocathode. Light 118, such as solar radiation, may be incident uponthe working electrode 108 as shown. The electrochemical cell 102 maythus be considered and configured as a photoelectrochemical cell. Insuch cases, illumination of the working electrode 108 may cause chargecarriers to be generated in the working electrode 108. Electrons thatreach the surface of the working electrode 108 may then be used in theCO₂ reduction. The photogenerated electrons may augment the electronsprovided via the current path. The photogenerated holes may move to thecounter electrode for the water oxidation. Further details regardingexamples of photocathodes are provided below.

The working electrode 108 includes a substrate 120. The substrate 120 ofthe working electrode 108 may constitute a part of an architecture, or asupport structure, of the working electrode 108. The substrate 120 maybe uniform or composite. For example, the substrate 120 may include anynumber of layers or other components. The substrate 120 thus may or maynot be monolithic. The shape of the substrate 120 may also vary. Forinstance, the substrate 120 may or may not be planar or flat.

The substrate 120 of the working electrode 108 may be active(functional) and/or passive (e.g., structural). In the latter case, thesubstrate 120 may be configured and act solely as a support structurefor a catalyst arrangement formed along an exterior surface of theworking electrode 108, as described below. Alternatively oradditionally, the substrate 120 may be composed of, or otherwiseinclude, a material suitable for the growth or other deposition of thecatalyst arrangement of the working electrode 108.

The substrate 120 may include a light absorbing material. The lightabsorbing material is configured to generate charge carriers upon solaror other illumination. The light absorbing material has a bandgap suchthat incident light generates charge carriers (electron-hole pairs)within the substrate. Some or all of the substrate 120 may be configuredfor photogeneration of electron-hole pairs. To that end, the substrate120 may include a semiconductor material. In some cases, the substrate120 is composed of, or otherwise includes, silicon. For instance, thesubstrate 120 may be provided as a silicon wafer. The silicon may bedoped. In some cases, the substrate 120 is heavily n-type doped, andmoderately or lightly p-type doped. The doping arrangement may vary. Forexample, one or more components of the substrate 120 may be non-doped(intrinsic), or effectively non-doped. The substrate 120 may includealternative or additional layers, including, for instance, support orother structural layers. In other cases, the substrate 120 is not lightabsorbing. In these and other cases, one or more other components of thephotocathode may be configured to act as a light absorber. Thus, inphotoelectrochemical cases, the semiconductor material may be configuredto generate charge carriers upon absorption of solar (or other)radiation, such that the chemical cell is configured as aphotoelectrochemical system.

The substrate 120 of the working electrode 108 establishes a surface atwhich a co-catalyst arrangement 124 of the electrode 108 is provided asdescribed below.

The working electrode 100 includes an array of conductive projections122 supported by the substrate 120. Each conductive projection 122 isconfigured to extract the charge carriers (e.g., electrons) from thesubstrate 120. The extraction brings the electrons to external sitesalong the conductive projections 122 for use in the CO₂ reduction. Insome cases, each conductive projection 122 is configured as a nanowire.Each conductive projection 122 may include a semiconductor core. In somecases, the core is or otherwise includes Gallium nitride (GaN). Othersemiconductor materials may be used, including, for instance, otherGroup III-V nitride semiconductor materials. The core of each nanowireor other conductive projection may be or include a columnar,post-shaped, or other elongated structure that extends outward (e.g.,upward) from the plane of the substrate 120. The semiconductor nanowiresmay be grown or formed as described in U.S. Pat. No. 8,563,395, theentire disclosure of which is hereby incorporated by reference. Theconductive projections 122 may be referred to herein as nanowires withthe understanding that the dimensions, size, shape, composition, andother characteristics of the projections 122 may vary.

Each conductive projection 122 has a semiconductor composition forcatalytic conversion of carbon dioxide (CO₂) in the chemical cell 102into, e.g., methane. As mentioned above, the semiconductor compositionmay include Gallium nitride. Additional or alternative semiconductormaterials may be used, including, for instance, indium nitride, indiumgallium nitride, aluminum nitride, boron nitride, aluminum oxide,silicon, and/or their alloys.

The conductive projections 122 may facilitate the conversion in one ormore ways. For instance, each conductive projection 122 may beconfigured to extract the charge carriers (e.g., electrons) generated inthe substrate 120. The extraction brings the electrons to external sitesalong the conductive projections 122 for use in the CO₂ reduction. Thecomposition of the conductive projections 122 may also form an interfacewell-suited for reduction of CO₂, as explained below.

Each conductive projection 122 may be or include a columnar,post-shaped, or other elongated structure that extends outward (e.g.,upward) from the plane of the substrate 120. The dimensions, size,shape, composition, and other characteristics of the conductiveprojections 122 may vary. For instance, each conductive projection 122may or may not be elongated like a nanowire. Thus, other types ofconductive projections from the substrate 120, such as various shapednanocrystals, may be used.

In some cases, one or more of the conductive projections 122 isconfigured to generate electron-hole pairs upon illumination. Forinstance, the conductive projections 122 may be configured to absorblight at frequencies different than other light absorbing components ofthe electrode 108. For example, one light absorbing component, such asthe substrate 120, may be configured for absorption in the visible orinfrared wavelength ranges, while another component may be configured toabsorb light at ultraviolet wavelengths. In other cases, the conductiveprojections 122 are the only light absorbing component of the electrode108.

The electrode 108 further includes a catalyst arrangement 124 disposedalong each conductive projection 122 for the reduction of carbon dioxide(CO2) in the chemical cell. The catalyst arrangement 124 includes acopper-based catalyst and an iron-based catalyst for the reduction ofcarbon dioxide (CO₂) in the chemical cell. In the example of FIG. 1, thecatalyst arrangement 124 includes a plurality of catalyst particles 126disposed across each conductive projection 122, and a distribution 128of an iron-based catalyst disposed adjacent to the plurality of catalystparticles. Each catalyst particle 126 includes copper. The iron-basedcatalyst may be composed of, or otherwise include, iron oxide. A Cu—Feco-catalyst arrangement is thus provided. In some cases, thecopper-based catalyst includes a plurality of copper nanoparticles. Thecopper-based catalyst may be disposed between the iron-based catalystand the conductive projection 122.

The copper-based catalyst and the iron-based catalyst may be linked byone or more metallic bonds, as schematically shown in FIGS. 1 and 5. ASchottky junction may be established between the catalyst arrangementand the semiconductor composition of the array of conductive projections122.

As described in the examples below, the catalyst arrangement may have aniron-to-copper ratio of about 6.3 to 1. Other ratios may be used,including other ratios in which the number of iron-based catalysts isgreater than the number of the copper-based catalysts. In some cases,the copper-based catalyst may be partially oxidized.

The distribution of the co-catalyst arrangement 124 across theconductive projections 122 may be uniform or non-uniform. For instance,the Cu-based particles 126 may thus be distributed randomly across theouter surface of each conductive projection 122. The arrangement shownin FIGS. 1 and 5 is for ease in illustration.

FIG. 2 depicts a method 200 of fabricating an electrode of anelectrochemical system in accordance with one example. The method 200may be used to manufacture any of the working electrodes describedherein or another electrode. The method 200 may include additional,fewer, or alternative acts. For instance, the method 200 may or may notinclude one or more acts directed to preparing a substrate (act 202).

The method 200 may begin with an act 202 in which a substrate isprepared. The substrate may be or be formed from a p-n Si wafer. In oneexample, a 2-inch Si wafer may be used, but other (e.g., larger) sizewafers may be used. Other semiconductors and substrates may be used.Preparation of the substrate may include one or more thermal diffusionor other doping procedures. In some cases, the act 202 may include twoor more doping procedures to establish an n⁺ layer or region, a p⁻ layeror region, and a p⁺ layer or region, as shown in the example of FIG. 2.

In the example of FIG. 2, the method 200 includes an act 204 in whichGaN or other nanowire arrays (or other conductive projections) are grownor otherwise formed on the substrate. The nanowire growth may beachieved in an act 206 in which plasma-assisted molecular beam epitaxyis implemented. The act 206 may be implemented under nitrogen-richconditions. In one example, the growth conditions were as follows: agrowth temperature of 790° C. for 1.5 h, a Ga beam equivalent pressureof about 6×10⁻⁸ Torr, a nitrogen flow rate of 1 standard cubiccentimeter per minute (sccm), and a plasma power of 350 W. The nanowiresprovide platforms or other structures for the co-catalyst arrangementdeposited in the following steps. Other platforms or structures may beformed.

In an act 208, a catalyst arrangement is deposited along each nanowireor other conductive projection of the electrode. The catalystarrangement includes a copper-based catalyst and an iron-based catalystfor the reduction of carbon dioxide (CO₂) in the chemical cell, asdescribed herein. The act 208 may include implementation of a number ofelectrodeposition cycles in an act 210, after which the structure isdried in an act 212. For example, the number of cycles may be about 10,but the number may vary. The act 210 may include immersing the array ofconductive projections in a solution, the solution including a copperprecursor and an iron precursor. Alternative or additional depositionprocedures may be used. Further details regarding examples of theelectrodeposition are provided below.

In some cases, the method 200 includes an act 214 in which the electrodeis annealed. One example electrode was annealed at 400° C. for 10 min informing gas (5% H₂, balance N₂) at a flow rate of 200 sccm. Theparameters of the anneal process may vary.

FIG. 4 depicts the results of the method 200 in accordance with oneexample. SEM images of GaN nanowires are shown before and afterelectrodeposition of the copper-based and iron-based co-catalysts.

Details regarding photoelectrochemical (PEC) performance of theco-catalyst arrangement, e.g., the binary Cu—Fe electrocatalyst, of thedisclosed PEC electrodes for the selective reduction of CO₂ to CH₄ arenow provided in connection with FIGS. 5-9.

Density functional theory (DFT) calculations reveal that theco-catalysts Cu and Fe in the binary system work in synergy to induce asignificantly distorted O—C—O angle of 126.05° from its original linearconfiguration at the interface to render a strong interaction with CO₂,and a drastic reduction in the reaction energy barrier, thus greatlyfacilitating methane synthesis. Experimentally, the Cu—Fe binaryelectrocatalyst is shown to exhibit high current density of −38.3 mAcm⁻² for silicon-based photoelectrodes with high Faradaic efficiency ofup to 51% and high TOF of 2176 h⁻¹ for PEC CO₂ reduction toward CH₄under simulated solar light (AM 1.5 G, 100 mW cm⁻²) at −1.2 V versusreversible hydrogen electrode (RHE), which is superior to that of bothCu and Fe catalyst individually. In addition, in some cases, thephotocathode may be made entirely of earth-abundant materials byindustrial semiconductor manufacturing processes, thereby presenting apromising route for producing clean fuels in aqueous solution usingsolar energy.

Further details regarding the mechanism(s) of CO₂ adsorption and/oractivation over Cu(111) and Fe_(x)O_(y)/Cu(111) are now described.Because the initial activation of the inert CO₂ is used for thesubsequent reactions, CO₂ adsorption characteristics were firstinvestigated using DFT calculations. As iron appears to be in itsoxidation state, Fe_(x)O_(y) was used in the analysis. The preferredorientation of Cu surface with the lowest surface energy, i.e., Cu(111)was adopted. Therefore, an inverse hydrogenated Fe₃O₆H₆/Cu(111) wasutilized as a representative model for Cu—Fe electrocatalyst, by takingthe aqueous CO₂ reduction environment and the preferable H spilloverfrom metal particles to oxide support into consideration. Illustrated inFIGS. 1 and 5 are the optimized structures for CO₂ adsorption onFe₃O₆H₆/Cu(111). For the case of CO₂ on Cu(111), CO₂ remains in theoriginal linear configuration with the O—C—O angle of 179.67°, and thetwo C—O bond lengths are similar to the CO₂ isolated gas-phase. On theother hand, it is found that for the case of CO₂ at the Fe₃O₆H₆/Cu(111)interface, the C atom strongly binds to the Cu atom underneath, with abond length of 1.98 Å; and one O atom attaches to the Fe atom with ashorter bond length of 1.96 Å. This signifies a much stronger bonding,which results in a significant distortion of CO₂ away from its originallinear form to a bent form with an O—C—O angle of 126.05°. A bidentateconfiguration is therefore formed, which facilitates the subsequentreactions. In addition, the interaction of CO₂ with the Fe₃O₆H₆/Cu(111)interface weakens the two C—O bonds of CO₂, leading to elongated C—Obonds (1.28 and 1.25 Å, respectively) from the original bond length of1.18 Å in an isolated CO₂. The weakened C—O bonds and the distorted CO₂configuration together highlight an activation of CO₂ upon chemisorptionat the interface, which is in stark contrast with the negligibleactivation of CO₂ on Cu(111) that is highly beneficial for CO₂reduction. The CO₂ activation mechanism at the Fe_(x)O_(y)/Cu(111)interface presents a situation in which an under-coordinated Fe atom atthe edge of the oxide cluster (i.e., essentially an O vacancy) acts asthe active center to bind one of the O atoms in CO₂. The coexistence ofiron oxides and Cu nanoparticles facilitates the formation of thebifunctional Fe_(x)O_(y)/Cu(111) interface. On one hand, theFe_(x)O_(y)/Cu(111) interface allows multiple adsorption sites anddirectly participates in stabilizing the key reaction intermediates,such as *CO₂, *C_(x)H_(y)O_(z), and *C_(x)H_(y). On the other hand, thestrong interaction between iron oxides and Cu nanoparticles results in aunique electronic structure that differs from those of isolatedcomponents, which is suitable for CO₂ activation and its subsequenttransformation. These results are consistent with the observation inthermal CO₂ catalysis at the metal/oxide interface, and are furtherverified by a CO₂ adsorption capacity measurement showing a much largerCO₂ adsorption capacity for the CuFe-based catalyst arrangements of thedisclosed electrodes, e.g., on GaN nanowires with a Si substrate,relative to that of Cu alone on GaN nanowires and a Si substrate.

The synthesis and characterization of the binary CuFe electrocatalystare now described in accordance with a number of examples. Inspired bythe theoretical results above, a binary CuFe catalyst is monolithicallyintegrated with GaN nanowire arrays on a planar n⁺-p silicon wafer,which may be achieved by combining highly controlled molecular beamepitaxy with facile electrodeposition. As illustrated in FIGS. 1-6,one-dimensional (1-D) GaN nanowires are first grown on planar n⁺-psilicon junction, e.g., with a length of about 300 nm and diametersvarying from about 30 to about 40 nm, using molecular beam epitaxy.Transmission electron microscope (TEM) images show that the GaNnanowires are nearly defect-free with lattice spacing of about 0.26 nm,suggesting the c-axis growth direction. Using these nanowires assupports, Cu-based and Fe-based catalysts were facilely co-deposited viaelectrocatalysis. After the electrodeposition, the morphology of the GaNnanowire arrays remains largely unchanged, as shown by comparing theimages depicted in Parts (a) and (b) of FIG. 4. Furthermore, scanningtransmission electron microscope-high angle annular dark-field(STEM-HAADF) images and elemental distribution mappings illustrate thatboth the Cu-based and Fe-based catalysts are clearly dispersed on theGaN nanowires with a unique alloyed geometry, as shown in the exampleimages of FIG. 6.

The loading of the binary CuFe catalyst arrangement may be optimized bythe one-dimensional GaN nanowires. For instance, 1-D nanostructures arefavorable for exposing the co-catalyst arrangement with high-densityactive sites. The ultrahigh surface-to-volume ratio of one-dimensionalnanostructures helps to reduce the loading amount of the catalyst. Theinductively coupled plasma-atomic emission spectrum (ICP-AES) indicatesthat the content of the binary CuFe catalyst arrangement is 0.041μmol·cm⁻² with an Fe/Cu ratio of 6.3/1. X-ray photoelectron spectroscopy(XPS) measurement was conducted to further analyze the chemical statesof Cu and Fe. The characteristic peaks of Cu 2p 3/2 and Cu 2p 1/2 appearat 933.2 eV and 953.1 eV, as shown in Part (h) of FIG. 6, due tometallic copper and/or partially oxidized copper. Meanwhile, the peaksof about 711 eV and 725 eV are associated with Fe 2p 3/2 and Fe 2p 1/2,respectively, as shown in Part (i) of FIG. 6. These peaks originate fromiron oxides and/or hydroxides (Fe_(x)O_(y)/Fe_(x)(OH)_(y)). An X-raydiffraction spectrum measurement illustrates that only a featured peakof GaN (002) at about 34° was observed for both GaN/Si and CuFecatalysts on GaN nanowires on a Si substrate. This may originate fromboth the low content of Cu and Fe and their amorphous phase, which agreewell with TEM and ICP-AES characterizations. The amorphous copper-ironcatalyst supported on one-dimensional GaN nanowire arrays may providesufficient surface defects as well as a large number of low-coordinatedatoms of the catalyst. Consequently, abundant active sites can beproduced for CO₂ reduction.

The photoelectrochemical CO₂ reduction reaction using the CuFeco-catalyst arrangement of the disclosed electrodes is now described inconnection with a number of examples. The PEC CO₂ reduction performanceof the CuFe catalyst arrangement on GaN nanowires on a Si substrate(hereinafter “CuFe@GaN NWs/Si”), as well as other photocathodes, wasexamined in CO₂-saturated 0.5 mol/L of KHCO₃ aqueous solution. As shownin Part (a) of FIG. 7, among all five of the tested photocathodes,CuFe@GaN NWs/Si exhibits the best J-V curve under standard one-sunillumination. Compared to bare n⁺-p silicon junction, GaN nanowires on aSi substrate shows an evidently improved J-V curve with an onsetpotential of −0.33 V (corresponding to a current density of −0.1 mAcm⁻²), but still suffers from rapid surface recombination and slowreaction kinetics because of the lack of catalysts. The introduction ofcatalysts may significantly improve the J-V behavior. The binary CuFecatalyst shows an enhancement compared to both Fe and Cu individually,confirming the synergetic effect of Cu and Fe for the reaction. Thesuperior onset potential of +0.23 V of the CuFe@GaN NWs/Si structure is200 mV and 290 mV higher than that of nanowires with either Fe or Cualone, i.e., “Fe/GaN NWs/Si” and “Cu/GaN NWs/Si,” respectively. Thecurrent density of the CuFe@GaN NWs/Si structure reaches −38.3 mA cm⁻²at −1.2 V, which is close to the light-limited current of thesilicon-based photocathode (about −45 mA cm⁻²) under one-sunillumination. Such improved performance may arise primarily from theCuFe catalyst offering active centers to promote the reaction kinetics.Moreover, photoluminescence spectra illustrates that the featured peakintensity for the various nanowire arrangements decreased in thefollowing order of GaN NWs/Si>Cu/GaN NWs/Si>CuFe@GaN NWs/Si. Itindicates that a Schottky junction is formed between the loadedcocatalysts and the GaN semiconductor, which is capable of greatlypromoting the electron-hole separation. Furthermore, the dramaticreduction in photoluminescence intensity of the CuFe@GaN nanowires ascompared to Cu/GaN nanowires suggests that the binary CuFe catalyst ismore favorable than Cu catalyst to promote electron-hole separation inGaN nanowires. Additionally, the light intensity affected the J-V curvesignificantly. The current density increased with the increasingintensity because more electron-hole pairs could be formed underillumination with higher intensity. In contrast, there is nearly nocurrent observed in the dark during the entire potential range examined.These results suggest that light-driven generation of electron-holepairs is a useful step for CO₂ reduction. Moreover, control experimentsconfirm that the LSV behavior under CO₂ atmosphere is superior to thatunder argon atmosphere, which further suggests the strong adsorption andactivation of CO₂ over the binary CuFe catalyst. Based on Faradaicefficiency measurements, both the GaN nanowire on Si substrate andFe/GaN nanowire on Si substrate arrangements do not produce any methane,as shown in Part (b) of FIG. 7. Hydrogen was the main byproduct with atrace amount of CO (Faradaic efficiency <1%). Although Cu iscatalytically active for methane synthesis, the Cu/GaN nanowire/Sisubstrate arrangement only shows a low Faradaic efficiency of about 20%.In stark contrast, the binary CuFe catalyst arrangement of the disclosedelectrodes gives rise to more than a two-fold improvement in Faradaicefficiency to 51% with a high current density of −38.3 mA cm⁻². As aconsequence, the partial current density of the CuFe@GaN NWs/Sistructure for CH₄ formation is as high as −19.5 mA cm⁻², as shown inPart (c) of FIG. 7, which is remarkably higher than the previouslyreported silicon photocathode for PEC CO₂ reduction toward CH₄. Theoptimal productivity of the CuFe@GaN NWs/Si structure for CH₄ approaches88.8 μmol·h⁻¹·cm⁻², which is 3.7 times larger than that of the Cu/GaNnanowire/Si substrate arrangement, while the Fe/GaN nanowire/Sisubstrate arrangement did not show any productivity under the sameexperimental conditions, as shown in Part (d) of FIG. 7. These resultsundoubtedly suggest that the binary CuFe catalyst arrangement of thedisclosed electrodes plays a useful role in promoting the methaneproduction. Electronic properties evaluation of Cu using X-rayphotoelectron spectrum also demonstrates a considerable shift of about+0.3 eV. Cu 2p 3/2 was shifted from 932.9 to 933.2 eV by incorporatingFe species, suggesting that Cu in CuFe@GaN NWs/Si structure iselectron-deficient compared to that of a Cu/GaN nanowire/Si substratearrangement. Such a notable change in electronic properties maycontribute to tuning the catalytic properties of Cu, and thusfacilitates the CO₂ reduction reaction towards methane. It is noted thatthere is an optimized CuFe catalyst for maximum activity and methaneselectivity. At a low loading amount of about 0.033 μmol·cm⁻² with aFe/Cu ratio of 4.5/1, the active sites of the CuFe@GaN NWs/Si structureare insufficient for suppressing charge carrier surface recombinationand improving the kinetics, resulting in limited activity. However, at ahigher Fe/Cu ratio of 12.9/1 with CuFe overloading of 0.075 μmol·cm⁻²,the light absorption of the silicon semiconductor may be suppressed, andthe inherent catalytic activities would be lowered. Therefore, there maybe a useful loading amount of 0.041 μmol·cm⁻² with a Fe/Cu ratio ofabout 6.3/1, enabling optimal optical and catalytic activity for highlyefficient PEC CO₂ reduction toward CH₄.

The Faradaic efficiency may depend on the applied potentials. Testresults are illustrated in Part (e) of FIG. 7 for a number of examples.The applied potentials play a role in the Faradaic efficiency. The onsetof the CuFe@GaN NWs/Si structure for methane synthesis is −0.4 V with amethane Faradaic efficiency of 1.2%, which is more positive than that of−0.7 V for Cu alone. It reveals that a significantly lower driving force(by as much as 0.3 V) is involved for the binary CuFe catalyst for theCO₂ reduction reaction. The underlying cause is that the binary CuFecatalyst is capable of initially activating the stable CO₂ molecule andreducing the high energy barrier, which is in agreement with thetheoretical calculation. At potentials more positive than −0.4 V, thedriving force is sufficient for hydrogen production but not forovercoming the high energy barrier for methane synthesis. Methane wasthus not formed. As the potential shifts negatively, Faradaic efficiencyof CH₄ formation is continuously improved with the increasing drivingforce and approaches to a maximum of 51% at −1.2 V. A more negativepotential, however, leads to a mild reduction in Faradaic efficiency to42% because of the severe competition of hydrogen evolution under highoverpotential as well as the CO₂ mass transport limitation.

High turnover frequency (TOF) is one aspect of the disclosed electrodesand systems. As shown in FIG. 7, plot f, an appreciable TOF of 9.5 h⁻¹is achieved under standard one-sun illumination at the onset potentialof −0.4 V. The negative shift of potential results in increasing TOF. At−1.2 V, a maximum TOF, which is as high as 2176 h⁻¹, is achieved at ahigh current density of −38.3 mA cm⁻² and high Faradaic efficiency of upto 51% despite of a slight reduction at more negative potential. Herein,the superior TOF may originate from the unique synergy of Cu and Fe inthe binary catalytic system. Additionally, the pronouncedsunlight-absorption ability and efficient charge carrier extraction ofthe GaN/Si platform also play a role, as addressed hereinbelow.

Further details regarding CO₂ conversion at the interface ofFe_(x)O_(y)/Cu are now provided in accordance with a number of examples.To gain fundamental insights underlying the superior performance of thebinary CuFe-based catalyst arrangement, the reaction pathways, reactionintermediates, potential-determining steps (PDSs), and free energydiagrams of the catalytic CO₂ reduction to CH₄ on Fe₃O₆H₆/Cu(111) aredescribed in comparison to those on Cu(111). Part (a) of FIG. 8 showsthe optimized structures of adsorption configuration for eachintermediate on Cu(111) and Fe₃O₆H₆/Cu(111), respectively. On theFe₃O₆H₆/Cu(111), the interfacial sites directly participate in bindingand stabilizing all the reaction intermediates. The O-bound species (*Oand *OH) prefer to bind to reduced Fe²⁺ cation in the metal oxide withthe η¹-O_(Fe2+) configuration, while for C, O-bound species (speciesbound through both C and O, i.e., *COOH, *CO, *CHO, *CH₂O, and *CH₃O),the metal/oxide interfacial sites are favored with the η²-C_(Cu)O_(Fe2+) configuration. Consequently, the Fe₃O₆H₆/Cu(111) interfacialsites are useful for methane synthesis via stabilizing all theintermediates during the complex eight-electron/proton coupling transferprocess.

Part (a) of FIG. 8 depicts the free energy diagram of the lowest energypathways of CO₂ reduction on the Cu(111) and Fe₃O₆H₆/Cu(111) under zeroelectrode potential (U=0 V) respectively. For the case of Cu(111), theprotonation of CO species (i.e., *CO→*CHO) is the the potentialdetermining step(s) (PDS), exhibiting a free energy change of 0.85 eV.On the other hand, for CO₂ at the interface of Fe₃O₆H₆/Cu(111), the PDSremains the same, but with an appreciably reduced free energy change of0.51 eV. By increasing the stability of the *CHO species relative to*CO, the energy efficiency of PEC reduction of CO₂ on theFe₃O₆H₆/Cu(111) interface may surpass the pure metals, due to thevarious structure with complementary chemical properties in themetal/oxide interfacial sites that work in synergy to facilitate the CO₂reduction into CH₄. Meanwhile, Fe₃O₆H₆/Cu(111) may hinder furtherreaction steps toward oxygen reduction due to an increased free energychange associated with the proton/electron-transfer step of *OH (i.e.,*OH protonation to H₂O(g)), as shown in Part (a) of FIG. 8. For thisstep, the Cu(111) surface involves 0.14 eV, while the Fe₃O₆H₆/Cu(111)involves 0.33 eV. Nonetheless, it would not alter the PDSs of the CO₂reduction on the Cu(111) surface and Fe₃O₆H₆/Cu(111) interface with bothof them lying in the *CO/*CHO step.

Part (b) of FIG. 8 shows the corresponding free energy diagrams of CO₂reduction at applied electrode potentials of U=−0.85 and −0.51 V for theCu(111) and Fe₃O₆H₆/Cu(111), respectively. These two electrodepotentials are the voltages involved for eliminating the free energychange of the PDSs (*CO/*CHO). It illustrates that the CH₄-formingreaction from CO₂ might occur at −0.85 and −0.51 V (vs. RHE) on theCu(111) surface and Fe₃O₆H₆/Cu(111) interface, respectively. It suggeststhat for methane synthesis, the onset potential of Fe₃O₆H₆/Cu(111) is0.34 V more positive than that of Cu(111), which is in agreement withthe experimental results that the onset of the binary CuFe catalyst is0.3 V lower than that of Cu alone.

In addition to Fe₃O₆H₆/Cu(111), CO₂ reduction at other possiblehydrogenated Fe_(x)O_(y)/Cu interfaces, i.e., Fe₃O₃H₃/Cu(111) andFe₆O₇H₇/Cu(111), was investigated. The results show that the reactionenergetics on Fe₃O₃H₃/Cu(111) and Fe₆O₇H₇/Cu(111) are similar to that ofFe₃O₆H₆/Cu(111).

Additionally, to consider the effect of partial oxidization on Cu ascharacterized in the XPS data, a series of DFT calculations wereconducted by constructing iron oxide clusters with varying atomic ratiosof Fe, Cu, and O on the surface of partially oxidized Cu, i.e.,Fe_(x)O_(y)/Cu₂O(111), similar to the cases of Cu(111). A similarconclusion has been found on the Fe_(x)O_(y)/Cu₂O(111) interfaces, thatis, in spite of quantitative variations among different systems, thesimilar qualitative trend confirms the role of the Fe_(x)O_(y)/Cu orFe_(x)O_(y)/Cu₂O(111) interface in activating CO₂ and stabilizing thereaction intermediates to facilitate the CO₂ reduction for methanesynthesis. The CO₂ reduction on pristine Cu₂O(111) is bottlenecked byboth of the hydrogenation of *CO to *CHO and *OH to H₂O with a freeenergy change for PDS being 1.02 and 1.12 eV, respectively. In contrast,free energy change of the hydrogenation of *CO to *CHO has been loweredto 0.89, 0.76, and 0.63 eV on Fe₃O₃H₃/Cu₂O(111), Fe₃O₆H₆/Cu₂O(111), andFe₆O₇H₇/Cu₂O(111), respectively. And the free energy change for anotherPDS of hydrogenation of *OH to H₂O has also been decreased due to aselective destabilization for the reaction intermediate of *OH. Thereaction mechanism of Fe_(x)O_(y)H_(z)/Cu₂O(111) is presumably the sameas that of Fe_(x)O_(y)H_(z)/Cu(111), because all the reactionintermediates share similar adsorption configurations and react with theCu atoms on Cu₂O(111) surface.

Further details regarding the contribution and function of the GaNnanowires are now described in connection with a number of examples.Apart from the catalyst, the influence of the GaN nanowires on theexcellent performance is now described. In the absence of the GaNnanowires, CuFe on a Si substrate (“CuFe/Si arrangement”) exhibited aplanar morphology similar to that of a bare silicon substrate. Controlexperiments indicate that the J-V curve of the CuFe/Si arrangementwithout GaN nanowires is inferior to the CuFe@GaN NWs/Si structure underthe same conditions, as shown in Parts (a) and (b) of FIG. 9. Thecurrent density of CuFe/Si is only −1.3 mA cm⁻² at about −1.2 V, whichis lower by a factor of 29.5 than −38.3 mA cm⁻² for the CuFe@GaN NWs/Sistructure, as shown in Part (a) of FIG. 9. Meanwhile, the Faradaicefficiency of 16.6% for the CuFe/Si arrangement is also much lower thanthat of 51% measured for the CuFe@GaN NWs/Si structure. In consequence,the productivity of the CuFe/Si arrangement (0.9 μmol·cm⁻²·h⁻¹) is twoorders of magnitude less than of the CuFe@GaN NWs/Si structure (88.9μmol·cm⁻²·h⁻¹), as shown in Part (b) of FIG. 9. Both optical andelectronic properties further show the significant improvement caused bythe GaN nanowires. UV-Vis relative differential reflectance spectraanalysis showed that the GaN nanowires enhance the sunlight absorptionof the n⁺-p silicon junction in a long wavelength range due to the lighttrapping effect. In addition, the bandgaps of GaN and Si areapproximately aligned, and the GaN nanowires are nearly defect-free andhas high electron mobility. Under illumination, the photogeneratedelectrons are thus readily extracted from the n⁺-p silicon junction andfurther transferred to the deposited CuFe catalyst in the presence ofthe GaN nanowires, which is in agreement with electrochemical impedancespectroscopy measurements. Therefore, it is reasonably concluded thatthe GaN nanowires may be a useful candidate for accelerating thereaction by enhancing the optical and electronic properties.Furthermore, owing to the one-dimensional structure of the GaN nanowirearrays, the catalysis may be spatially decoupled from sunlightcollection and charge carriers' separation, as shown in Part (c) of FIG.9, which may maximize the synergy of Cu and Fe for methane synthesis byproviding sufficient active sites with high atom efficiency.

Isotopic experiments were also conducted to clarify that the methane wasproduced from CO₂ reduction. When the reaction was performed in C13labeled bicarbonate aqueous solution under the atmosphere of ¹³CO₂, gaschromatography-mass spectroscopy (GC-MS) analysis only showed a peak atm/z=17 resulting from ¹³CH₄. The formation of ¹²CH₄ was negligible. Incontrast, when the blank experiment was carried out in argon-purgedNa₂SO₄ aqueous solution, there was no methane synthesized. These resultssuggest that methane is produced from CO₂. Moreover, the discloseddevice is capable of exhibiting stable operation of 10 hours. Noelemental dissolution of the CuFe@GaN NWs/Si into the aqueous solutionwas found by ICP; and the morphology of the catalytic architectureremained unchanged, further confirming the stability of the device.

Further details regarding the fabrication of a binary CuFe catalystarrangement over GaN Nanowires (NWs) and a Si substrate (Si) byelectrocatalysis are now described in connection with a number ofexamples. A GaN NWs/Si structure was produced as the platform fordepositing the binary CuFe catalyst arrangement. A polished p-Si (100)wafer was doped using phosphorus and boron as n-type and p-type dopantsby spin coating, respectively. The doped silicon was then annealed at900° C. under argon atmosphere for 4 hours to produce an n⁺-p siliconjunction. The as-prepared n⁺-p silicon junction was further employed forplasma-assisted molecular beam epitaxial growth of GaN nanowires withgermanium as an n-type dopant. The growth was carried out at 790° C.under nitrogen-rich conditions with a nitrogen flow rate of 1.0 standardcubic centimeter per minute (sccm) for 1.5 hours. The Ga beam pressureis about 6×10⁻⁸ torr with a plasma power of 350 W.

In the electrodeposition procedure, the GaN NWs/Si structure wasimmersed into a three-electrode cell, in which Pt wire and Ag/AgCl wereused as counter electrode and reference electrode, respectively. A 200mL mixture of CuCl₂ (Sigma-Aldrich, ≥99%) and FeCl₂ (Alfa-Aesar, 99.5%)aqueous solution with desired concentrations was added into the chamber.The fabrication of an example of the CuFe-based catalyst arrangement onthe GaN NWs/Si structure with a Cu₁Fe_(6.3) ratio may use 0.1 mmol/LCuCl₂ and 0.01 mmol/L FeCl₂ as the precursors of the CuFe catalyst. Theelectrodeposition was conducted using cyclic voltammetry at thepotential range from +2.5 to −2.5 V versus Ag/AgCl. The number ofdepositing cycles was 10 with a scanning rate of 100 mV/s. The Fe/Curatio in the CuFe catalyst may be tailored by tuning the concentrationratio of FeCl₂ to CuCl₂ in the precursors' solutions while keeping theCuCl2 concentration of 0.1 mmol/L unchanged. The fabricatedphotoelectrodes were thoroughly rinsed with distilled water and driedwith air after the electrodeposition. Both of the Cu/GaN NWs/Si andFe/GaN NWs/Si arrangements may be produced using the same procedure,with the main difference being the precursors used. Moreover, the CuFecatalyst was electrochemically deposited on bare n⁺-p silicon junctionfor a comparison through the same procedure.

In summary, described above are electrodes and systems in which aninexpensive binary catalyst arrangement (e.g., CuFe catalyst) is coupledwith GaN nanowires (or other conductive projections) on a n+-p siliconwafer. The binary catalyst arrangement is highly active and selectivefor photoelectrochemical CO₂ reduction toward CH₄. Both experimental andtheoretical results indicate that Cu and Fe work in synergy forspontaneous CO₂ activation and conversion with severely deformed CO₂molecular structure and reduced reaction energy barrier by stabilizingkey reaction intermediates. As a result, a high current density of −38.3mA cm⁻² for a silicon-based photocathode with a high Faradaic efficiencyof 51% and a distinct turnover frequency of 2176 h⁻¹ is achieved formethane synthesis under simulated solar light. The device ismanufactured using earth-abundant materials and may be operated for atleast 10 hours without degradation. The disclosed electrodes and systemspresent a promising route for producing clean solar fuels fromphotoelectrocatalytic CO² reduction in an aqueous cell.

The present disclosure has been described with reference to specificexamples that are intended to be illustrative only and not to belimiting of the disclosure. Changes, additions and/or deletions may bemade to the examples without departing from the spirit and scope of thedisclosure.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom.

What is claimed is:
 1. An electrode of a chemical cell, the electrodecomprising: a substrate having a surface; an array of conductiveprojections supported by the substrate and extending outward from thesurface of the substrate, each conductive projection of the array ofconductive projections having a semiconductor composition for reductionof carbon dioxide (CO₂) in the chemical cell; and a catalyst arrangementdisposed along each conductive projection of the array of conductiveprojections, the catalyst arrangement comprising a copper-based catalystand an iron-based catalyst for the reduction of carbon dioxide (CO₂) inthe chemical cell.
 2. The electrode of claim 1, wherein the copper-basedcatalyst comprises a plurality of copper nanoparticles.
 3. The electrodeof claim 1, wherein the iron-based catalyst comprises a distribution ofiron oxide disposed in a co-catalyst arrangement with the copper-basedcatalyst.
 4. The electrode of claim 1, wherein the copper-based catalystis disposed between the iron-based catalyst and the conductiveprojection.
 5. The electrode of claim 1, wherein the copper-basedcatalyst and the iron-based catalyst are linked by a metallic bond. 6.The electrode of claim 1, wherein: the substrate comprises asemiconductor material; and the semiconductor material is configured togenerate charge carriers upon absorption of solar radiation such thatthe chemical cell is configured as a photoelectrochemical system.
 7. Theelectrode of claim 6, wherein the array of conductive projections areconfigured to extract the charge carriers generated in the substrate. 8.The electrode of claim 1, wherein each conductive projection of thearray of conductive projections comprises a respective nanowire.
 9. Theelectrode of claim 1, wherein each conductive projection of the array ofconductive projections comprises a Group III-V semiconductor material.10. The electrode of claim 1, wherein the structure is planar.
 11. Theelectrode of claim 1, wherein the semiconductor composition of the arrayof conductive projections establishes a Schottky junction with thecatalyst arrangement.
 12. The electrode of claim 1, wherein the catalystarrangement has an iron-to-copper ratio of about 6.3 to
 1. 13. Theelectrode of claim 1, wherein the copper-based catalyst may be partiallyoxidized.
 14. The electrode of claim 1, wherein the chemical cell is athermochemical cell.
 15. An electrochemical system comprising a workingelectrode configured in accordance with the electrode of claim 1, andfurther comprising: a counter electrode; an electrolyte in which theworking and counter electrodes are immersed; and a voltage source thatapplies a bias voltage between the working and counter electrodes;wherein the bias voltage establishes a preference for the reduction ofcarbon dioxide (CO₂) at the working electrode toward methane.
 16. Aphotocathode for a photoelectrochemical cell, the photocathodecomprising: a substrate comprising a light absorbing material, the lightabsorbing material being configured to generate charge carriers uponsolar illumination; an array of conductive projections supported by thesubstrate, each conductive projection of the array of conductiveprojections being configured to extract the charge carriers from thesubstrate; a plurality of catalyst particles disposed across eachconductive projection of the array of conductive projections, eachcatalyst particle of the plurality of catalyst particles comprisingcopper; and a distribution of an iron-based catalyst disposed adjacentto the plurality of catalyst particles in a co-catalyst arrangement withthe plurality of catalyst particles for the reduction of carbon dioxide(CO₂) in the chemical cell.
 17. The photocathode of claim 16, whereinthe iron-based catalyst comprises iron oxide.
 18. The photocathode ofclaim 16, wherein each conductive projection of the array of conductiveprojections comprises a respective nanowire.
 19. The photocathode ofclaim 16, wherein each catalyst particle of the plurality of catalystparticles is configured as a copper nanoparticle.
 20. The photocathodeof claim 16, wherein each conductive projection of the array ofconductive projections comprises a Group III-V semiconductor material.21. A photoelectrochemical system comprising a working photocathodeconfigured in accordance with the photocathode of claim 16, and furthercomprising: a counter electrode; an electrolyte in which the workingphotocathode and the counter electrode are immersed; and a voltagesource that applies a bias voltage between the working photocathode andthe counter electrode; wherein the bias voltage establishes a preferencefor the reduction of carbon dioxide (CO₂) at the working electrodetoward methane.
 22. A method of fabricating an electrode of anelectrochemical system, the method comprising: growing an array ofconductive projections on a semiconductor substrate, each conductiveprojection of the array of conductive projections having a semiconductorcomposition for reduction of carbon dioxide (CO₂) in the electrochemicalsystem; and depositing a catalyst arrangement along each conductiveprojection of the array of conductive projections, the catalystarrangement comprising a copper-based catalyst and an iron-basedcatalyst for the reduction of carbon dioxide (CO₂) in the chemical cell.23. The method of claim 22, wherein depositing the catalyst arrangementcomprises implementing a number of electrodeposition cycles.
 24. Themethod of claim 23, wherein the number of electrodeposition cycles isabout 10 cycles.
 25. The method of claim 23, wherein implementing thenumber of electrodeposition cycles comprises immersing the array ofconductive projections in a solution, the solution comprising a copperprecursor and an iron precursor.